Supercapacitors (Ultra-Capacitors or Electro-Chemical Capacitors):
Supercapacitors are being considered for electric vehicle (EV), renewable energy storage, and modem grid applications. The high volumetric capacitance density of a supercapacitor derives from using porous electrodes to create a large surface area conducive to the formation of diffuse double layer charges. This electric double layer (EDL) is formed in the electrolyte near the electrode surface when voltage is imposed (FIGS. 1(B) and 2(A)). The required ions for this EDL mechanism near an electrode are pre-existing in the liquid electrolyte when the cell is made or in a discharged state (FIG. 2(B)), and do not come from the opposite electrode surface or interior. In other words, the required ions to be formed into an EDL near the surface of a negative electrode (anode) active material (e.g., activated carbon particle) do not come from the bulk or surface per se of the positive electrode (cathode). The required ions (for the anode EDL formation during the cell charging step) are not captured or stored earlier in the surface or interior of a cathode active material (instead, they are present inside the electrolyte phase at either the anode side or the cathode side). Similarly, the required ions to be formed into an EDL near the surface (but not on the surface) of a cathode active material do not come from the very surface or interior of an anode active material. Furthermore, the number of cations and the number of anions that participate in the charge storage function are essentially equal in a supercapacitor.
When the supercapacitor is re-charged, the ions (both cations and anions) that are already in the liquid electrolyte are electrochemically driven to form EDLs near their respective electrodes. There is no major exchange of ions between an anode active material and a cathode active material. The amount (capacitance) of charges that can be stored is dictated solely by the concentrations of cations and anions that are already available in the electrolyte. These concentrations are typically very low (limited by the solubility of a salt in a solvent), resulting in a low energy density. Further, lithium ions are usually not part of preferred or commonly used supercapacitor electrolytes. When the supercapacitor is discharged, both the cations and the anions are simply re-distributed in the electrolyte in a random manner (FIG. 2(B)).
In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some electrochemical reactions (e.g., redox). In such a pseudo-capacitor, the ions involved in a redox pair also pre-exist in the electrolyte. Again, there is no major exchange of ions between an anode active material and a cathode active material.
Since the formation of EDLs does not involve a chemical reaction or an exchange of ions between the two opposite electrodes, the charge or discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (typically 3,000-8,000 W/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 10-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Lithium-ion batteries possess a much higher energy density, typically in the range of 100-180 Wh/kg, based on the cell weight.
Lithium-Ion Batteries:
Although possessing a much higher energy density, lithium-ion batteries deliver a very low power density (typically 100-500 W/Kg), requiring typically hours for re-charge. Conventional lithium-ion batteries also pose some safety concern.
The low power density or long re-charge time of a lithium ion battery is due to the mechanism of shuttling lithium ions between the interior of an anode and the interior of a cathode, which requires lithium ions to enter or intercalate into the bulk of anode active material particles during re-charge, and into the bulk of cathode active material particles during discharge. For instance, as illustrated in FIG. 1(A), in a most commonly used lithium-ion battery featuring graphite particles as an anode active material, lithium ions are required to diffuse into the inter-planar spaces of a graphite crystal at the anode during re-charge. Most of these lithium ions have to come all the way from the cathode side by diffusing out of the bulk of a cathode active particle, through the pores of a solid separator (pores being filled with a liquid electrolyte), and into the bulk of a graphite particle at the anode. The liquid electrolyte (where lithium ions can easily swim through) is excluded from the bulk of a particle and, hence, the migration of lithium ions from the particle surface to the interior (e.g. the center of a solid graphite particle) must occur via very slow solid-state diffusion (intercalation), as illustrated at the bottom portion of FIG. 1(A).
During discharge, lithium ions diffuse out of the anode active material (e.g. de-intercalate out of graphite particles), migrate through the liquid electrolyte phase, and then diffuse into the bulk of complex cathode crystals (e.g. intercalate into particles lithium cobalt oxide, lithium iron phosphate, or other lithium insertion compound). In other words, liquid electrolyte only reaches the external surface of a solid particle (e.g. graphite particle 10 μm in diameter) and lithium ions swimming in the liquid electrolyte can only migrate (via fast liquid-state diffusion) to the graphite surface. To penetrate into the bulk of a solid graphite particle would require a slow solid-state diffusion (commonly referred to as “intercalation”) of lithium ions. The diffusion coefficients of lithium in solid particles of lithium metal oxide are typically 10−16-10−8 cm2/sec (more typically 10−14-10−10 cm2/sec), and those of lithium in liquid are approximately 10−6 cm2/sec.
In other words, these intercalation or solid-state diffusion processes require a long time to accomplish because solid-state diffusion (or diffusion inside a solid) is difficult and slow. This is why, for instance, the current lithium-ion battery for plug-in hybrid vehicles requires 2-7 hours of recharge time, as opposed to just seconds for supercapacitors. The above discussion suggests that an energy storage device that is capable of storing as much energy as in a battery and yet can be fully recharged in one or two minutes like a supercapacitor would be considered a revolutionary advancement in energy storage technology.
More Recent Developments:
Recently, chemically functionalized, multi-walled carbon nano-tubes (CNTs) containing carbonyl groups were used by Lee, et al as a cathode material for a lithium-ion capacitor (LIC, as illustrated in FIG. 1(E)) containing lithium titanate as the anode material [S. W. Lee, et al, “High Power Lithium Batteries from Functionalized Carbon Nanotubes,” Nature Nanotechnology, 5 (2010) 531-537]. In a super-battery configuration (FIG. 1(D)), lithium foil was used as the anode and functionalized CNTs as the cathode, providing a relatively high power density. However, the CNT-based electrodes prepared by the layer-by-layer (LBL) approach suffer from several technical issues beyond just the high costs. Some of these issues are:                (1) CNTs contain a significant amount of impurity, particularly those transition metal or noble metal particles used as a catalyst required of a chemical vapor deposition process for CNT production. These catalytic materials are highly undesirable in a battery electrode due to their high propensity to cause harmful reactions with electrolyte.        (2) CNTs tend to form a tangled mass resembling a hairball, which is difficult to work with during electrode fabrication (e.g., difficult to disperse in a liquid solvent or resin matrix).        (3) The so-called “layer-by-layer” approach (LBL) used by Lee, et al is a slow and expensive process that is not amenable to large-scale fabrication of battery electrodes, or mass production of electrodes with an adequate thickness (most of the batteries have an electrode thickness of 100-300 μm). The thickness of the LBL electrodes produced by Lee, et al (a noted MIT research group) was limited to 3 μm or less.        (4) One might wonder how the thickness of the LBL CNT electrodes would impact the performance of the cells. A careful inspection of the data provided by Lee, et al (e.g. FIG. S-7 of the Supporting Material of Lee, et al) show that the power density dropped by one order of magnitude when the LBL CNT electrode thickness was increased from 0.3 μm to 3.0 μm. The performance is likely to drop even further if the electrode thickness is increased to that of a useful battery or supercapacitor electrode (e.g., 100-300 μm).        (5) Although the ultra-thin LBL CNT electrodes provide a high power density (since Li ions only have to travel an extremely short distance), there was no data to prove that CNT-based electrodes of practical thickness could even work due to the poor CNT dispersion and electrolyte inaccesability issues. Lee, et al showed that the CNT-based composite electrodes prepared without using the LBL approach did not exhibit good performance.        (6) CNTs have very limited amounts of suitable sites to accept a functional group without damaging the basal plane structure. A CNT has only one end that is readily functionalizable and this end is an extremely small proportion of the total CNT surface. By chemically functionalizing the exterior basal plane, one could dramatically compromise the electronic conductivity of a CNT.        
Most recently, our research groups have reported, in two patent applications, the development of two new classes of highly conducting cathode active materials having a functional group that is capable of rapidly and reversibly forming a redox reaction with lithium ions. These materials are nano graphene (both single-layer graphene and multi-layer graphene sheets, collectively referred to as nano graphene platelets or NGPs) and disordered carbon (including soft carbon, hard carbon, carbon black, activated carbon, amorphous carbon, etc). These two patent applications are: C. G. Liu, et al., “Lithium Super-battery with a Functionalized Nano Graphene Cathode,” U.S. patent application Ser. No. 12/806,679 (Aug. 19, 2010) and C. G. Liu, et al, “Lithium Super-battery with a Functionalized Disordered Carbon Cathode,” U.S. patent application Ser. No. 12/924,211 (Sep. 23, 2010).
These new types of cathode active materials (used in the so-called “lithium super-battery”) include a chemically functionalized nano graphene platelet (NGP) or a functionalized disordered carbon material having certain specific functional groups capable of reversibly and rapidly forming a redox pair with a lithium ion during the charge and discharge cycles of a battery cell. In these two patent applications, the functionalized disordered carbon or functionalized NGP is used in the cathode (not the anode) of the lithium super-battery. In this cathode, lithium ions in the liquid electrolyte only have to migrate to the edges or surfaces of graphene sheets (in the case of functionalized NGP cathode), or the edges/surfaces of the aromatic ring structures (small graphene sheets) in a disordered carbon matrix. No solid-state diffusion is required at the cathode. The presence of a functionalized graphene or carbon having functional groups thereon enables reversible storage of lithium on the surfaces (including edges), not the bulk, of the cathode material. Such a cathode material provides one type of lithium-storing or lithium-capturing surface.
In conventional lithium-ion batteries, lithium ions must diffuse into and out of the bulk of a cathode active material, such as lithium cobalt oxide (LiCoO2) and lithium iron phosphate (LiFePO4). In these conventional lithium-ion batteries, lithium ions must also diffuse in and out of the inter-planar spaces in a graphite crystal serving as an anode active material. The lithium insertion or extraction procedures at both the cathode and the anode are very slow. Due to these slow processes of lithium diffusion in and out of these intercalation compounds (commonly referred to as solid-state diffusion or intercalation processes), the conventional lithium ion batteries do not exhibit a high power density and the batteries require a long re-charge time. None of these conventional devices rely on select functional groups (e.g. attached at the edge or basal plane surfaces of a graphene sheet) that readily and reversibly form a redox reaction with a lithium ion from a lithium-containing electrolyte.
In contrast, the super-battery as reported in our two earlier patent applications (U.S. application Ser. No. 12/806,679 and Ser. No. 12/924,211), relies on the operation of a fast and reversible reaction between a functional group (attached or bonded to a graphene structure at the cathode) and a lithium ion in the electrolyte. Lithium ions coming from the anode side through a separator only have to diffuse in the liquid electrolyte to reach a surface/edge of a graphene plane in the cathode. These lithium ions do not need to diffuse into or out of the volume of a solid particle. Since no diffusion-limited intercalation is involved at the cathode, this process is fast and can occur in seconds. Hence, this is a totally new class of hybrid supercapacitor-battery that exhibits unparalleled and unprecedented combined performance of an exceptional power density, high energy density, long and stable cycle life, and wide operating temperature range. This device has the best of both battery and supercapacitor worlds.
In the lithium super-batteries described in these two earlier patent applications, the anode comprises either particles of a lithium titanate-type anode active material (Type-2 super-battery, still requiring some solid state diffusion in the first discharge or first charge operation, but no intercalation thereafter) or a lithium foil alone, along with an anode current collector (Type-1 super-battery). Hence, these lithium super-batteries are also referred to as partially surface-mediated, lithium ion-exchanging cells.
The instant application claims the benefits of the two co-pending U.S. application Ser. No. 12/806,679 (Aug. 19, 2010) and Ser. No. 12/924,211 (Sep. 23, 2010) Ser. No. 12/928,927, but discloses a more general and versatile approach that also involves the exchange of massive lithium ions between an anode and the surfaces of a cathode. These cathode surfaces in the instant application are not based on a functionalized material (defined as a material bearing a functional group capable of forming a redox pair with lithium). Instead, we have most surprisingly observed that, without any functional group, some graphene surfaces are fully capable of capturing or trapping more lithium atoms. Regardless if the surfaces contain functional groups or not, graphene surfaces are capable of storing lithium atoms in a stable and reversible manner, provided these surfaces are accessible to lithium ion-containing electrolyte and are in direct contact with the electrolyte. After extensive in-depth studies, we have further observed that the lithium storing capacity is in direct proportion to the total surface area that is directly exposed to the lithium ion-containing electrolyte. For instance, some of the specific capacity measurements were conducted on the cells containing a pristine graphene cathode composed of essentially all carbon atoms only (>99% C), having no functional group such as —OH or —COOH. Hence, the mechanism of Li-functional group redox reaction could not be the lithium storage mechanism. The two co-pending US patent applications claim the functionalized material-based super-batteries, but the instant application claims the super-batteries based on a non-functionalized material cathode.